The outbreak has led to several recalls of salads made at a Fresh Express production facility in Streamwood, Illinois that were sold at major retailers Walmart, Hy-Vee, Aldi and Jewel-Osco, USA TODAY reported.

"[O]bviously there was some breakdown in the quality chain," Rutgers University food microbiologist Donald W. Schaffner told The New York Times. He said the size and spread of the outbreak suggested "some rather significant sanitary breakdown in the production of this food."

The parasite Cyclospora cayetanensis causes an infection called cyclosporiasis when humans eat or drink contaminated food or water, according to the CDC. The main symptom is watery diarrhea, but it can also cause stomach cramping, appetite loss and fatigue. It can last from a few days to longer than a month and is typically treated with antibiotics.

The first major foodborne cyclosporiasis outbreak was in the mid-1990s, Schaffner told The New York Times, and no one knows exactly what caused it. The current outbreak marks the third year in a row that there has been an outbreak of the illness during the warmer months.

"It's likely due to the quality of the water used to irrigate the produce, and it probably has something to do with human fecal contamination of that water, but of course there's a whole lot of unknowns," Schaffner said. "Very often with these fresh produce outbreaks, we never learn the definitive cause."

Illnesses from the current outbreak have been reported in Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota and Wisconsin between May 11 and June 17, the CDC said. So far, the CDC and Food and Drug Administration (FDA) investigation has uncovered a Fresh Express bagged salad mix containing iceberg lettuce, carrots and red cabbage as the likely culprit.

The outbreak has already prompted a number of recalls.

1. Brand: Marketside brand Classic Iceberg Salad

Store: Walmart

States: Iowa, Michigan, Minnesota, Nebraska, North Dakota, South Dakota and Wisconsin

Bag Size: 12 to 24 ounces

Use By: 05/19/2020 through 07/04/2020

2. Brand: Little Salad Bar brand Garden Salad

Store: ALDI

States: Arkansas, Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, North Dakota, South Dakota and Wisconsin

Bag Size: 12 ounces

Use By: 05/01/2020 through 06/29/2020

3. Brand: Hy-Vee brand Garden Salad

Store: Hy-Vee

States: Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, South Dakota and Wisconsin

Bag Size: 12 ounces

Use By: All dates

4. Brand: Jewel-Osco Signature Farms brand Garden Salad

Store: Jewel-Osco

States: Illinois, Indiana and Iowa

Bag Size: 12 ounces

Use By: 05/16/2020 through 07/04/2020

Then, on Saturday, Fresh Express issued a voluntary recall of all the salads made at its Streamwood facility that contain iceberg lettuce, red cabbage or carrots. The recall only impacts salads with a "Z" at the beginning of the Product Code in the upper-right-hand corner of the front of the package. The recalled salads have a Product Code of Z178 or lower.

"Our immediate thoughts and concern are for those consumers who have become ill due to the outbreak," Fresh Express said in a statement Saturday reported by USA Today. "Out of an abundance of caution, we have issued a voluntary recall of both branded and private label salad products that were produced at the Streamwood facility and contain those ingredients."

The Fresh Express recall came after the FDA began an investigation of the Streamwood plant and raised concerns that some products not already recalled might be impacted, according to the FDA.

"Consumers should not eat, and restaurants and retailers should not sell or serve any Fresh Express products currently on the market that were made in the Streamwood, Illinois, production facility and contain either iceberg lettuce, red cabbage, or carrots," the agency recommended.